-LRB- CNN -RRB- -- Nathan Cox-Reed has a toothache .

He thinks he needs a root canal , but the full-time student , 22 , is uninsured . He ca n't afford a trip to the dentist .

`` I 'm only working 30 hours a week . I would n't have enough money to do something like that , '' said Cox-Reed , a film and video student at Columbia College in Chicago .

While many young adults are now covered by the Affordable Care Act , able to remain on their parents ' insurance until age 26 , the rules are different for those like Cox-Reed , who grew up in the foster care system .

There are more than 400,000 children in foster care in the United States , the Department of Health and Human Services said last year . All are provided with health care coverage as long as they are wards of the state .

When foster kids turn 18 , they age out of the system and instantly lose their coverage .

That 's about to change , when another part of Obamacare takes effect on January 1 , 2014 . Medicaid coverage will be extended for former foster youth until they reach 26 , as long as the individual was in foster care and enrolled in Medicaid until the age of 18 .

`` I definitely think it would be a big relief , and I would definitely feel more secure as far as my health goes , '' Cox-Reed said .

But there 's a catch . Cox-Reed has dreams of traveling across the nation and becoming a filmmaker . A future relocation could jeopardize his medical coverage .

States will only be required to keep former foster children on Medicaid if they continue to reside in the state where they were in foster care originally .

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This part of the provision is `` an incredibly troubling aspect , '' said Washington attorney Brooke Lehmann , who founded the child and family advocacy group Childworks . Young adults can be highly mobile as they move for educational purposes , job opportunities and a host of other reasons , she said .

`` You ca n't be on a film set if you are uninsured , '' Cox-Reed said . `` You could get hurt . I definitely think -LSB- being uninsured -RSB- is limiting and it 's a letdown , because what if I do get a job out of state ? I might not be able to take it . ''

While `` it 's a great provision , '' said Joan Alker , co-executive director of the Georgetown Center for Children and Families and a professor at the Georgetown University Health Policy Institute , limiting extended Medicaid enrollment because of relocation could threaten the provision 's effectiveness .

She , Lehmann and other child advocates wrote a letter , circulated by the Children 's Defense Fund , to the Centers for Medicare & Medicaid Services at the Department of Health and Human Services , which is working on the revisions before this part of the ACA is implemented in 2014 , asking them to reconsider this limitation .

Even if this part of the ACA is reinterpreted , former foster children like Cox-Reed may face other challenges with the existing system -- namely , not all doctors and dentists accept Medicaid .

Vicki Rodriguez , 20 , who grew up in foster care in Illinois , found that out the hard way . She needs someone to remove her wisdom teeth , but ca n't find a single dentist who will take her Medicaid card and she ca n't afford to pay for it on her own .

`` My tooth on my left side is coming in sideways , and it 's pushing my other teeth forward , '' she said . `` I ca n't go anywhere because it could cost about $ 3,000 -LSB- including -RSB- the X-rays and everything else that they have to do before . ''

Rodriguez believes having coverage extended until she reaches 26 would be `` really awesome , '' but noted that even those with coverage are limited .

`` People think , ` Oh yeah , the foster kids have everything covered for them , they can get everything done with their medical card , ' but it 's not true , '' she said . `` There 's really a lot that we ca n't do . ''

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Lehmann recalls one of her past clients , who she said was `` actively psychotic and suicidal '' : It took two years for her to obtain medication , Lehmann said , because of the difficulty in finding a psychiatrist who would take Medicaid .

`` The reimbursement rates are abysmal '' for providers , she said .

Health and Human Services has not responded to the child advocates ' letter and did not respond to repeated requests for comment from CNN on the matter .

The advocates , including Alker and Lehmann , say enrollment in Medicaid should be faster and easier to ensure all eligible young adults are enrolled without obstacles .

An outreach campaign could inform eligible individuals so former foster youth are aware of the opportunity , they say .

`` States are still trying to figure out how these former foster youth can be identified , how they can let young adults know that this benefit is open to them and how they can enroll , '' Lehmann said .

CNN 's Jen Christensen contributed to this report .

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Foster kids currently age out of health coverage at 18

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A new Obamacare provision takes effect in January , extending coverage to age 26

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But moving out of state can end health coverage for former foster kids